A tercentenary is the three-hundredth anniversary of a significant event. Otherwise, the form occurs mainly in chemical names, but not commonly even then: the terphenyls contain three linked benzene rings; a chemical reaction that is termolecular involves three molecules; a tervalent chemical radical has a combining power of three; terbutaline (from elements of its formal chemical name, see also but-) is a drug used especially in the treatment of asthma. The form has largely been replaced by tri-.